Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask (=reticle) illuminated via the illumination system is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure onto the light-sensitive coating on the substrate.
Various approaches are known for specifically setting or correcting the polarization distribution in the illumination system or in the projection objective in order to optimize the imaging contrast. For example, in an imaging process with high apertures as can be realized in the immersion lithography, a preferred polarization distribution is configured such that the diffraction orders are tangentially polarized in the wafer plane in order to avoid a loss of contrast due to the so-called vector-effect. The expression “vector effect”, which occurs in imaging processes with high apertures, is used to describe the situation in which the vector of the electric field has, in the image region, different directions for different diffraction orders even if the polarization state is the same, which results from the fact that the p-polarized components (TM-components) of the vector of the electric field are no longer parallel to each other, so that imaging contrast depends on the polarization state.
WO 2005/069081 A2 discloses a polarization-influencing optical element which includes an optically active crystal and involves a thickness profile which varies in the direction of the optical axis of the crystal, whereby for example a constant linear input polarization distribution can be transformed into a tangential output polarization distribution (in which the preferred polarization direction or the oscillation direction of the vector of the electric field is oriented in perpendicular relationship with the radius directed towards the optical system axis).
There are also situations where an adaptation or adjustment of the illumination setting (of the intensity distribution obtained in a given plane, in particular in a pupil plane) is desired. Examples are the rotation of the illumination poles of a dipole or quadrupole illumination setting about the optical axis (the adjustment or realization of a “rotated” dipole or quadrupole-setting), which may be favourable or desired in order to produce an image of oblique or inclined mask structures.
FIGS. 7a-d schematically illustrate such an adaptation of the illumination setting to an oblique or inclined mask structure 725 (in which the repetitive direction is rotated through an angle a with respect to the y-axis in the coordinate system or a mask structure 715 having only horizontal structures, respectively). In the dipole illumination setting 720 according to FIG. 7c realized in the illumination device, the illumination poles 721, 722 are also rotated through an angle α with respect to the y-axis in order to achieve an adaptation or matching with the mask structure 725 shown in FIG. 7d. However, a rotation of only the intensity distribution results in a situation in which, for the dipole illumination setting 710 according to FIG. 7a, a quasi-tangential polarization distribution is no longer present in case of rotation of the illumination poles 720, 721 in the dipole illumination setting 720 according to FIG. 7c, leading to a loss of the afore described optimization of contrast.
U.S. Pat. No. 5,614,988 discloses, among other things, a projection exposure apparatus having a plurality of projection optical units, where a matching between images formed through the respective projection optical units is achieved by rotating reflective surfaces in the projection optical units about the optical axis.